This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display unit capable of producing visual images with the assistance of environmental light and/or back light.
The liquid crystal display devices are categorized in a reflection type and a transparent type. A difference between the reflection type and the transparent type is the light source. The reflection type liquid crystal display device is equipped with a reflection plate, but does not have any built-in light source. On the other hand, the transparent type liquid crystal display device has a built-in light source, which is usually called as xe2x80x9cback light sourcexe2x80x9d. Environmental light is incident on the reflection type liquid crystal display device, and is reflected on the reflection plate so that the liquid crystal produces a visual image with the assistance of the reflection light. On the other hand, the back light source radiates the back light, and the back light locally passes through the liquid crystal layer so that a visual images are produced.
The reflection type liquid crystal display device is thin, light and low power consumption, because any back light source is not required for the image production. These features are desirable for a portable electronic device such as a portable telephone. On the other hand, the transparent type liquid crystal display device can produce a visual image without the environmental light, and a clear visual image is produced thereon. In the following description, the image production without the back light is referred to as xe2x80x9creflection modexe2x80x9d, and the image production through the back light is referred to as xe2x80x9ctransmission modexe2x80x9d.
The common structure of the liquid crystal display devices includes a liquid crystal layer, a back light source or a reflection plate and a driving circuit. Various sorts of technologies have been employed in the liquid crystal layer, and are twisted nematic liquid crystal, which is usually abbreviated as xe2x80x9cTNxe2x80x9d, a single polarization plate technology, super twisted nematic liquid crystal, which is usually abbreviated as xe2x80x9cSTNxe2x80x9d, a guest-host technology, a polymer dispersed liquid crystal, which is usually abbreviated as xe2x80x9cPDLCxe2x80x9d and cholesteric phase liquid crystal. The driving circuit makes the liquid crystal layer locally transparent. An active matrix driving circuit is popular. The active matrix driving circuit includes switching elements, which are implemented by thin film transistors or MIM (Meal-Insulator-Metal) diodes, and defines a matrix of pixels in the liquid crystal layer. The active matrix driving circuit makes the pixels selectively transparent and non-transparent so that a fine visual image is produced on the liquid crystal display device. The light, which is radiated from the back light source or reflected on the reflection plate, passes through the transparent pixels so that the fine visual image is produced.
There is a compromise between the transparent type liquid crystal display device and the reflection type liquid crystal display device. The compromise is hereinafter referred to as xe2x80x9csemi-transparent type liquid crystal display devicexe2x80x9d. A typical example of the semi-transparent type liquid crystal display device is disclosed in Japan Patent No. 2955277, and is shown in FIG. 1.
The prior art semi-transparent type liquid crystal display device includes pixels arranged in rows and columns, and each pixel electrode 1 occupies a rectangular area. The pixel electrode 1 has a reflection area 5 made of non-transparent metal and a transparent area 6 made of indium-tin-oxide, i.e., ITO, and both areas 5/6 are defined in the lower substrate structure. Gate lines 2 extend in parallel to one another in a direction parallel to the long side lines of the rectangular area, and drain lines 3 extend in parallel to one another in a direction parallel to the short end lines of the rectangular area. Thin film transistors 4 are respectively associated with the pixel electrodes 1. The gate electrodes of the thin film transistors 4 are connected to the associated gate lines 2, and the drain electrodes of the thin film transistors 4 are connected to the associated drain lines 3. The pixel electrodes 1 are connected to the source electrodes of the thin film transistors 4.
While the semi-transparent type liquid crystal display device is producing a visual image on the matrix of the pixels in the light, the back light source turns off, and the environment light is reflected on the reflection area 5 for producing the visual image. When a user brings the semi-transparent type liquid crystal display device in a dark room, the back light source turns on, and the back light passes through the transparent area 6 so that the visual image is clearly produced on the matrix of pixels. Thus, the environment light and back light are selectively used for the semi-transparent type liquid crystal display device. The semi-transparent type liquid crystal display device consumes the electric power the amount of which is less than that of the transparent type liquid crystal display device, and produces a clear image in the dark space.
A problem is encountered in the prior art standard semi-transparent type liquid crystal display device in that the output light intensity is hardly optimized due to the difference in length of optical path between the incident light and the back light. In other words, the retardation between the incident light and the back light is not ignoreable. In order to solve the problem, an insulting layer 8 is provided under the reflection area 5, and the reflection plate 9 is formed under the insulating layer 8 in the prior art semi-transparent liquid crystal display device disclosed in Japan Patent No. 2955277 as shown in FIG. 2. The insulating layer 8 makes the gap dr narrower than the gap df so as to cancel the different in length of optical path.
In order to maximize the luminance in the transmitted light in the semi-transparent type liquid crystal display device, the ECB mode, in which the twist angle is zero, is preferable. If the twist angle is 72 degrees in the transmission mode, the transmitted light available for the visual image is only 50%. When the twist angle is reduced to zero, the transmitted light available for a visual image is increased to 100%. Although the reflectance is maximized in the range between 2 microns and 3 microns in the reflection mode on the condition that the twist angle is 72 degrees, the reflectance is peaked at dr=1.5 microns on the condition that the twist angle is reduced to zero as shown in FIG. 3. For this reason, the wavelength dispersion is widened at zero degree rather than at 72 degrees. The visual image becomes yellowish when the gap is increased from the optimum gap. On the other hand, if the gap is decreased from the optimum gap, the visual image becomes bluish. Especially, the insulating layer 8 makes the surface of the lower substrate structure rolled, and, accordingly, the thickness of the liquid crystal is varied. The difference between the mean thickness and the maximum/minimum thickness is of the order of 0.3 micron. FIG. 4 shows chromaticity coordinates. In the chromaticity coordinates, the mean thickness of the liquid crystal is changed from 1.4 microns through 1.7 microns to 2.0 microns. When the mean thickness is 1.7 microns, the chromaticity is at (0.33, 0.35). When the liquid crystal is maximized at 2.0 microns thick, the chromaticity is changed to (0.37, 0.38), and the visual image is reddened. On the other hand, when the liquid crystal is minimized at 1.4 microns thick, the chromaticity is changed to (0.30, 0.32), and the visual image becomes yellowish. This is because of the fact that the peak reflectance of red, green and blue light components is offset as shown in FIG. 5. When the liquid crystal layer is less in thickness than 1.5 microns, at which the reflectance of the green light component is peaked, the reflectance of the red light component is rapidly reduced. On the other hand, when the liquid crystal layer is greater in thickness than 1.5 microns, the blue light component is rapidly reduced. Thus, the difference in reflectance among the red, green and blue light components makes the chromaticity varied.
The ECB mode and a VA mode, which will be described hereinbelow, are attractive to the manufacturers for liquid crystal display devices of the next generation. However, a problem is encountered in that the trial is varied in the reflection area 5 due to the wavelength dispersion. The problem is inherent in the prior art semi-transparent liquid crystal display device disclosed in Japan Patent No. 2955277, because the liquid crystal layer is unavoidably varied at xc2x10.3 micron thick.
It is therefore an important object of the present invention to provide a reflection type/semi-transparent type liquid crystal display device, which is small in wavelength dispersion, large in luminance and low in power consumption without sacrifice of the luminance.
In accordance with one aspect of the present invention, there is provided a reflection type or semi-transparent type liquid crystal display device comprising two substrate structures spaced from each other and including electrodes selectively formed therein so as to be selectively applied with a minimum potential difference and a maximum potential difference for creating local electric fields and color filters formed in one of the two substrate structures and a liquid crystal layer confined in the space between the two substrate structures and locally changed between transparent state and non-transparent state in the presence of the local electric fields for producing a color visual image, an the thickness of the liquid crystal layer and one of the minimum and maximum potential differences are determined in such a manner that a reflectance of the liquid crystal layer to one of the red, green and blue light components has an extreme value when the one of the minimum and maximum potential differences is applied between selected ones of the electrodes.
In accordance with another aspect of the present invention, there is provided a semi-transparent type liquid crystal display device comprising a first substrate structure including signal lines, thin film transistors selectively connected to the signal lines so as to be selectively changed between on-state for propagating data signals and off-state, reflecting electrodes connected to the associated thin film transistors for receiving the data signals in the on-state and transparent electrodes respectively paired with the reflecting electrodes and connected to the associated thin film transistors for receiving the data signals in the on-state, a second substrate structure including a counter electrodes opposed to the pairs of reflecting and transparent electrodes for creating local electric fields and a liquid crystal layer confined between the first substrate and the second substrate and applied with a minimum potential difference and a maximum potential difference between the pairs of reflecting and transparent electrodes and the counter electrode for partially becoming transparent in the presence of the local electric fields, and the liquid crystal molecules of the liquid crystal layer between the reflecting electrodes and the counter electrode and between the transparent electrodes and the counter electrode are oriented in a direction inclined from both of the horizontally oriented state and the vertically oriented state by a certain angle equal to or greater than 10 degrees in the presence of one of the minimum and maximum potential differences.
In accordance with yet another aspect of the present invention, there is provided a semi-transparent type liquid crystal display device comprising two substrate structures selectively formed with electrodes and having reflecting regions for reflecting light incident thereonto and transparent regions for passing back light and a liquid crystal layer confined in a space between the two substrate structures and selectively applied with a minimum potential difference and a maximum potential difference so as to be locally changed between transparent state and non-transparent state for producing a color visual image, and a gap in the transparent regions and one of the minimum and maximum potential differences are determined in such a manner that a reflectance of the liquid crystal layer to one of the red, green and blue light components has an extreme value.